Insulin plays a central role in the regulation of carbohydrate, fat, and protein metabolism in the body. Diabetes mellitus (commonly referred to simply as diabetes) is a disease characterized by disregulation of metabolism, particularly glucose metabolism. In normal individuals, a rise in blood glucose levels (such as that which occurs immediately following eating) triggers the islet beta cells of the pancreas to secrete insulin, a peptide hormone, into the bloodstream. The insulin binds to insulin receptors located on a number of cell types, notably muscle cells, and thereby signals the cells to increase the rate of glucose uptake into the cells. As the blood glucose returns to normal pre-prandial levels, the amount of insulin in the blood also drops. In the absence of insulin, blood glucose levels would rise to dangerously high levels (a condition termed hyperglycemia), possibly resulting in death. Too much insulin causes abnormally low blood glucose levels (hypoglycemia), which is also dangerous and possibly fatal. In a normal individual, built-in feedback loops regulating the secretion of insulin and its clearance from the systemic circulation prevent both hyperglycemic and hypoglycemic conditions from occurring.
Diabetes mellitus is a disease affecting about 3% of the population of Sweden. Of these 3%, approximately 20% suffer from Type I diabetes, and the remainder from Type II diabetes.
Type I diabetes, or insulin-dependent diabetes mellitus (IDDM), usually begins in childhood. It is characterized by atrophy of the pancreatic beta cells, resulting in a decrease or cessation of insulin production, and leaving the patient dependent on exogenous insulin for survival.
The more common Type II diabetes, or non-insulin-dependent diabetes mellitus (NIDDM), generally occurs in patients older than 40 years. These patients may, at least initially, have normal or even high levels of insulin in their blood, but exhibit an abnormally low rate of cellular uptake of glucose in response to insulin. Although Type II diabetes often can be treated by controlling the patient's diet, administration of exogenous insulin to supplement that secreted by the patient's beta cells may also prove necessary.
Insulin cannot be orally administered in effective doses, since it is rapidly degraded by enzymes in the gastrointestinal tract and low pH in the stomach before it can reach the bloodstream. The standard method of administration is by subcutaneous injection of an isotonic solution of insulin, usually by the patient him/herself. The necessity for injection causes a great deal of inconvenience and discomfort to many sufferers, and local reactions can occur at the injection site. In addition there is an abnormal, non-physiological, plasma concentration profile for injected insulin. This abnormal plasma concentration profile is undesirable and increases the risk of side effects related to the long term treatment of diabetes.
Because of these disadvantages, there is a need for insulin in a form which is administrable other than by injection. In attempts to produce such different forms of insulin, various proposals have been made. For example, products for nasal, rectal and buccal administration have been suggested, with much effort being concentrated on products for nasal administration. Nasal administration is however problematic and permits only a very low bioavailability. Pulmonary delivery of systemically active drugs has gained increasing interest over the last years, and some investigations have included the pulmonary delivery of insulin. Most of these are concerned with solutions or suspensions for pulmonary delivery for example by nebulisers and pressurised metered dose inhalers, and all have met with limited success.